1. Field of the Invention
This invention relates to a process for producing a negative-charging electrophotographic photosensitive member which can reduce image defects and maintain good image formation over a long period of time, and also relates to a negative-charging electrophotographic photosensitive member and an electrophotographic apparatus.
2. Related Background Art
Materials that form photoconductive layers in solid-state image pick-up devices or in electrophotographic photosensitive members in the field of image formation or in character readers are required to have properties as follows: They are highly sensitive, have a high SN ratio [photocurrent (Ip)/dark current (Id)], absorption spectra suited to spectral characteristics of electromagnetic waves to be applied, high response to light, and desired dark resistance, and are harmless to human bodies when used, and also in the solid-state image pick-up devices, have properties of easily erasing afterimages in a prescribed period of time. In particular, in the case of electrophotographic photosensitive members used in business machines in offices, harmlessness in use is important.
Materials that attract notice from such viewpoints include amorphous silicon (hereinafter “a-Si”) whose dangling bonds have been modified with monovalent elements such as hydrogen or halogen atoms, and it is applied to electrophotographic photosensitive members.
As processes by which electrophotographic photosensitive members composed of a-Si are formed on conductive substrates, many processes are known in the art, as exemplified by sputtering, a process in which source gases are decomposed by heat (thermal CVD), a process in which source gases are decomposed by light (photo-assisted CVD) and a process in which source gases are decomposed by plasma (plasma-assisted CVD). In particular, the plasma-assisted CVD (chemical vapor deposition), i.e., a process in which source gases are decomposed by direct-current or high-frequency or microwave glow discharge to form deposited films on the conductive substrate has been put into practical use in a very advanced state at present in the field of processes of forming electrophotographic photosensitive members. As the layer construction of such deposited films, the following are proposed: electrophotographic photosensitive members composed primarily of a-Si and modification elements added appropriately, as conventionally done, and in addition thereto those constructed to have an upper-part blocking layer or a surface protective layer, having blocking power, which is further deposited on the surface side (see, e.g., Japanese Patent Application Laid-open No. H08-15882). This Japanese Patent Application Laid-open No. 08-15882 discloses a photosensitive member provided between a photoconductive layer and a surface protective layer with an upper-part blocking layer having carbon atoms in a smaller content than the surface protective layer and incorporated with atoms capable of controlling conductivity.
The a-Si films have such a disposition that, where any dust of the order of micrometers have adhered to the substrate surface, the films may undergo abnormal growth on the dust serving as nuclei during film formation and protuberances come to grow. These protuberances cause image defects on images. In order to prevent such image defects, a technique is proposed in which the vertexes of protuberances present on the photosensitive member surface after film formation are flattened by polishing (see, e.g., Japanese Patent Application Laid-open No. 2001-318480). This Japanese Patent Application Laid-open No. 2001-318480 discloses a post-treatment method in which an electrophotographic photosensitive member is held and rotated and, while a polishing tape wound around an elastic roller and the surface of the photosensitive member are brought into pressure contact, the polishing tape is allowed to travel, carrying out polishing to flatten the protuberances of the photosensitive member surface.
An example of the protuberances is shown in FIG. 1. Each protuberance 111 has the shape of a reversed cone whose vertex starts from dust 110, and has a disposition that it lowers electrical resistance because there are a very large number of localized levels at an interface 112 between a normal deposited portion and the protuberant portion, and allows electric charges to pass through the interface 112 toward the substrate side. Hence, the portion at which a protuberance is present appears as a white dot in solid black images (in the case of reverse development, appears as a black dot in solid white images). This image defect called “dot” has hitherto not been regarded as a defective depending on size even when several dots are present in an A3-size sheet. However, further improvement in quality is required where the photosensitive member is to be mounted to a color copying machine, and even when only one dot is present in a A3-size sheet, it regarded as a defective. Such protuberances are originated from dust, and hence substrates to be used are strictly cleaned before films are formed thereon, where the steps of setting the substrates in a film forming apparatus are all operated in a clean room or in vacuum. In this way, efforts have been made so as to lessen as far as possible the dust which may adhere to the substrate surface before the film formation is started, and a certain effect have been obtained.
However, the cause of the occurrence of protuberances is not only the dust having adhered to the substrate surface. That is, where a-Si electrophotographic photosensitive members are produced, the layer thickness required is as very large as several micrometers to tens of micrometers, and hence the film formation time reaches several hours to tens of hours. During such film formation, the a-Si becomes deposited not only on the substrates, but also on walls of the film forming furnace and on components inside the film forming furnace. The deposits on the furnace walls and components are not filmy ones deposited on the substrate but powdery deposits. In some cases, they may have weakly adhered to cause film come-off during film formation carried out over a long time. Once even any slight film has come off during film formation, it causes dust, and the dust adheres to the surface of a photosensitive member under deposition so that starting from the dust, the protuberances come about which are abnormal-growth portions. Accordingly, in order to maintain a high yield, polishing is carried out to flatten the protuberances formed by abnormal growth, and an upper-part blocking layer having the ability to block the acquired electric charges is so deposited as to cover the flattened protuberances to prevent such a phenomenon that the acquired electric charges leak through protuberant portions or the interfaces between normal portions and the protuberant portions. Such a measure has been taken, and a certain effect have been obtained (see, e.g., Japanese Patent Application Laid-open No. 2004-133396).
As methods for charging a-Si photosensitive members electrostatically, they include a corona charging system making use of corona charging, a roller charging system making use of a conductive roller to perform charging by direct discharge, and an injection charging system in which the contact area is sufficiently taken up using magnetic particles or the like and electric charges are directly injected to the photosensitive member surface to perform charging. In particular, the corona charging system and the roller charging system make use of discharge, and hence discharge products tend to adhere to the photosensitive member surface. In addition, the a-Si photosensitive members have a surface layer having much higher hardness than organic photosensitive members and the like, and hence the discharge products are apt to remain on the surface, so that the discharge products and water content may combine due to the adsorption of water content in a high humidity environment to bring the surface into a low resistance, where electric charges at the surface tend to move to cause a phenomenon of image deletion in some cases. Accordingly, it has been necessary in some cases to take various measures on how to rub the surface, how to manage the temperature of photosensitive members, and so forth.
On the other hand, the injection charging system is a charging system in which any discharge is not intentionally used and electric charges are directly injected from the part coming into contact with the photosensitive member surface, and hence it can not easily cause the phenomenon such as image deletion. Also, the injection charging system, which is a contact charging system, is of a voltage control type, while the corona charging system is of a current control type, and the former has such an advantage that any non-uniformity of charge potential can be rendered relatively small. In a conventional injection charging system, a contact charging member having particles in the form of a magnetic brush, composed of a magnetic material and magnetic particles, is brought into contact with the photosensitive member surface to achieve the improvement of charging performance (see, e.g., Japanese Patent Application Laid-open No. H08-6353).